The present invention relates to capacitor grade tantalum powders. Such powders are produced by various chemical and electrolytic reduction processes, a principal form of practice of chemical processing being as described in U.S. Pat. No. 2,950,185 to Hellier and Martin. In that processing, potassium fluotantalate double salt is reduced with sodium. The reduction takes place in a molten bath of such salt, per se, or mixed with a diluent salt of sodium chloride, or other alkali metal halides. The quantity of sodium added is normally less than the theoretical amount for complete reaction according to the reduction equation: EQU K.sub.2 TaF.sub.7 + Na .fwdarw. Ta + 5NaF + 2KF
sodium stoichiometries between 90%-98% are preferred for the production of low oxygen content tantalum powders, which are formed in the molten bath. After reduction, the molten bath is resolidified by cooling. Salt byproducts of the reduction reaction freeze in and around loosely aggregated powder layers and in strata separate from metal-powder-containing layers to comprise an aggregate spongy structure which can be milled to fine size and leached to remove soluble components therein, leaving an insoluble tantalum powder. The powder can be further processed by size segregation, further leaching, and the like. Such powders are sintered into compacts which are formable into capacitor anodes or the like. In some instances in the art, the powders are presintered and milled to produce agglomerates which can be sintered to produce capacitor anodes or the like.
There are several rigid chemical and physical criteria for the quality of the tantalum powders to be usable in producing high quality capacitor anodes and the like. Low oxygen content (below about 1,500 ppm) is essential. During the course of production of the tantalum powders, including the double salt and reducing agent molten bath stage (involving temperatures of 900.degree.-1200.degree. C.) to produce the primary powder, oxygen pick up from processing atmospheres, from impure starting materials and from equipment are ever-present dangers. Where the oxygen is not readily removed in the course of final leaching steps, the resultant high oxygen powder must be recycled or dedicated to a substantially derated usage or scrapped. All three alternatives represent considerable economic loss and waste of a scarce resource since known and practicably available sources of tantalum are limited in relation to present and anticipated demand.
It is therefore an important object of the invention to provide a method for recovering tantalum of high oxygen content to a usable, non-derated form.
It is a further object of the invention to achieve foregoing objective in practical and economic fashion maintaining control over powder size distribution.
It is a further object of the invention to achieve one or more of the preceding objects without undue dangers of reintroducing oxygen or other contamination.